1. Field of Invention
The present invention relates to a quadrature voltage controlled oscillator (VCO), and more particularly, to a quadrature VCO circuitry with symmetrical spiral inductors and differential varactors implemented with complementary MOS in a system on chip.
2. Description of the Related Art
Voltage controlled oscillator (VCO) is widely used, for example in many transceivers incorporate quadrature downconverions mixers. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links, etc. In the design of these transceiver systems, a major challenge is the generation of quadrature local oscillating (LO) signals.
Referring to FIG. 1, a differential pair incorporating two auxiliary cross-coupled differential pairs in one of conventional arts is illustrated herein. The main differential pair 103 includes transistors 112 and 114. The gates of 112 and 114 are coupled to a differential input voltage Vi1 and Vi2. The sources of 112 and 114 are coupled to a current source. The sources of 112 and 114 are coupled to a 114. The gates of 112 and 114 are coupled to a differential input coupled to ground. Current source Iss is typically constructed as known to those skilled in the art. The drains of 112 and 114 supply currents I1 and I2 respectively. The drains of 112 and 114 are coupled to current outputs I1 and I2, respectively.
The first auxiliary differential pair 107 includes transistors 116 and 118. The gate of 116 is coupled to differential input voltage Vi1. The gate of 118 is coupled to differential input voltage Vi2. The sources of 116 and 118 are coupled together and then to a first terminal of a first current source Iss/n. A second terminal of Iss/n is coupled to a ground potential. Current source Iss/n is typically constructed as a conventional current sources as is known to those skilled in the art. The drain of 116 is coupled to the drain of 114. The drain of 118 is coupled to the drain of 112.
The second auxiliary differential pair 109 includes transistors 126 and 128. The gate of 126 is coupled to differential input voltage Vi1. The gate of 128 is coupled to differential input voltage Vi2. The sources of 126 and 128 are tied together to a first terminal of a second current source Iss/n. A second terminal of Iss/n is coupled to ground. The drain of 126 is coupled to the drain of 114. The drain of 128 is coupled to the drain of 112. From system point of view, quadrature LO signals are required to achieve high phase accuracy, good gain matching and low phase noise.
Referring to FIG. 3B, it is a plan view of a multi-track spiral inductor suitable for integration onto an integrated circuit, such as one produced with a CMOS process. A square spiral inductor that is wound in two turns with several narrow tracks disposed in parallel upon a substrate. Equivalently any number of tracks may be used to achieve a multi-track design. A turn is counted each time the track is wound around in spiral such that a starting point is passed. Typically 5 to 20 turns are utilized in a spiral, with 3 to 10 producing optimum performance.
Referring to FIG. 4B, a cross sectional view of a varactor is illustrated. A single-ended varactor is provided according to the connecting method shown therein, where control signal Vctrl is coupled to the n+ diffusion area in the N-well, and the single-ended voltage V+/V− is read out from coupling to the p+ diffusion area in the N-well. According to the arrangement and connection of the diffusion area in FIG. 4B, two reverse-biased diodes are common-anode coupled for forming a varactor.
In light of aforementioned technologies of prior art, a differential amplifying pair and a LC circuit consisted of symmetrical laid-out spiral inductors and differential varactors are desired, since differential structures improve the quality factor, and the symmetry of the circuit lowers flicker noise up-conversion. Moreover, a differential LC circuit reduces the chip area consumed thereby.